DSpace at VNU: Crystalline evolution and large coercivity in Dy-doped (Nd, Dy)(2)Fe14B alpha- Fe nanocomposite magnets

Home Search Collections Journals About Contact us My IOPscience Crystalline evolution and large coercivity in Dy-doped (Nd,Dy)2Fe14B/α-Fe nanocomposite magnets This content has been downloaded from IOPscience Please scroll down to see the full text 2007 J Phys D: Appl Phys 40 119 (http://iopscience.iop.org/0022-3727/40/1/001) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 141.161.91.14 This content was downloaded on 07/09/2015 at 15:31 Please note that terms and conditions apply INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J Phys D: Appl Phys 40 (2007) 119–122 doi:10.1088/0022-3727/40/1/001 Crystalline evolution and large coercivity in Dy-doped (Nd,Dy)2Fe14B/α-Fe nanocomposite magnets N D The1,2 , N Q Hoa1,3 , S K Oh3 , S C Yu3 , H D Anh1 , L V Vu1 and N Chau1,4 Center for Materials Science, College of Science, Vietnam National University Hanoi, 334 Nguyen Trai Road, Hanoi, Vietnam Department at Physics and Astronomy, University of Glasgow, Glasgow C12 8QQ, UK Department of Physics, Chungbuk National University, 361-763 Cheongju, Korea E-mail: chau@cms.edu.vn Received April 2006, in final form November 2006 Published 15 December 2006 Online at stacks.iop.org/JPhysD/40/119 Abstract Nanocomposite hard magnetic materials (Nd,Dy)4.5 Fe77.5 B18 (No 1) and (Nd,Dy)4.5 Fe76 B18 Nb1.2 Cu0.3 (No 2) have been prepared by crystallizing amorphous ribbons, fabricated by single roll melt-spinning The evolution of a multiphase structure was monitored by an x-ray diffractometer and by thermomagnetic measurement We observed that, at annealing temperatures below 670 ◦ C, there is crystallization of soft phase Fe3 B and a small amount of hard phase Nd2 Fe14 B At annealing temperatures above 670 ◦ C, crystallization of α-Fe and probably Dy2 Fe14 B phases with large magnetocrystalline anisotropy led to a drastic enhancement in the hard magnetic properties of the materials The maximum value of HC is found to be 4.2 kOe for sample No For sample No 2, with co-doping of Nb and Cu, nanostructure refinement yields a strong enhancement in exchange coupling between the component phases Thereby, we obtained high reduced-remanence of 0.78, high remanence of 1.15 and a high (BH)max value up to 16.2 MGOe (Some figures in this article are in colour only in the electronic version) Introduction Nanocomposite exchange-spring magnets provide an alternative way of producing high remanence magnetic materials, which can be used to make resin bonded magnets Additional merit is in the cost reduction owing to the low consumption of rare-earth elements Nanocomposite magnets have been studied for compositions like (Pr,Nd)2 Fe14 B/Fe3 B [1, 2] and (Pr,Nd)2 Fe14 B/α-Fe(Co) [3–8] Some of the recently reported new kind of nanocomposite magnets are self-assembled FePt [9, 10], melt-spun nanocomposite magnets FePtB [11, 12] and nanocomposite (Nd,Dy)(Fe,Co,Nb,B)5.5 /α-Fe multilayer magnets [13] So far, high-performance nanocomposite magnets have not been obtained with low rare-earth content because Author to whom any correspondence should be addressed 0022-3727/07/010119+04$30.00 © 2007 IOP Publishing Ltd high coercivity has not been reached Nonetheless, the effect of doping elements on the microstructure and magnetic properties of nanocomposite magnets has shown something remarkable [8, 12–15] With a small amount of Cr and Co doping, a special microstructure, namely the cellular structure, was observed in α-Fe(Co)/Nd2 Fe14 B nanocomposite magnets [8] In fact, the formation of a cellular structure resulted in high shape anisotropy of nano-grains, which contributes to the total magnetic anisotropy of the material Thereby, a high-performance nanocomposite magnet was obtained with a very low concentration of Nd (4.5 at.%) Hence, the role played by the cellular structure could be an important ingredient that should be taken into account in producing high-performance magnets with low rare-earth content Therefore, in this article, we investigate further the effect of substituting a small amount of Dy for Nd and the role of Nb and Cu in microstructural refinement Printed in the UK 119 N D The et al Experimental The amorphous precursors with the composition of (Nd,Dy)4.5 Fe77.5 B18 (No 1) and (Nd,Dy)4.5 Fe75.5 B18.5 Nb1.2 Cu0.3 (No 2) have been fabricated by the rapid-quenching technique in an Ar atmosphere in an Edmund Buehler melt-spinner with a linear speed of 30 m s−1 Subsequently, we put the amorphous flakes in a quartz tube, evacuated to a high vacuum state, then filled the tube with highly purified Ar and finally annealed them isothermally at appropriate temperatures The crystalline evolution of as-cast samples was monitored using a differential scanning calorimeter (TA Instruments model 2960) The structure of the samples was examined by an x-ray diffractometer (Bruker model D5005) with Cu–Kα radiation Microstructural observation was carried out by a scanning electron microscope (JEOL model 5410 LV) Magnetic characteristics were measured by a vibrating sample magnetometer (Model DMS 880) with the maximum applied field of 13.5 kOe, and demagnetization curves were measured using a hysteresisgrapher (Walker model AMH 25) The demagnetizing factor of the specimens was approximately corrected Results and discussion Figure 1(a) displays differential scanning calorimetry (DSC) results for amorphous ribbons with a heating rate of 20 ◦ C min−1 The curves exhibit three clearly exothermal peaks, which are related to the formation of a magnetic phase in the thermal process According to Li et al [16], the crystalline evolution of (Nd,Dy)FeB amorphous ribbons could be expressed as: Amorphous → Amorphous’ + o-Fe3 B → Amorphous” + t-Fe3 B + (Nd,Dy)2 Fe14 B → t-Fe3 B + (Nd,Dy)2 Fe14 B + α-Fe However, structural examination by an x-ray diffractometer (XRD (figure 1(b))) shows a different result It can be described as follows: Figure DSC curves of as-cast samples with the heating rate of 20 ◦ C min−1 measured in flowing Ar gas (a) and XRD results for sample No at different annealing temperatures (b) • The first peak corresponds to the crystallization of the Fe3 B and Nd2 Fe14 B phases, which is similar to that of other NdFeB-based amorphous ribbons [5, 8]; • The second peak, occurring at a slightly higher temperature, is related to the formation of α-Fe, and seems to be a (Nd,Dy)2 Fe14 B phase (suggestion); • In sample No 2, the exothermal peaks shift to lower temperature (see figure 1) because of the doping of Cu with low melting temperature and a high diffusion coefficient as the nucleation is accompanied in crystallization [17] A multiphase structure is also confirmed by measuring the thermomagnetic curve of the annealed samples (see figure 2) Obviously, the curves exhibit Curie temperatures of the Nd2 Fe14 B and Fe3 B phases As seen in figure 2, the thermomagnetic curve of the sample annealed at 650 ◦ C, which is lower than the temperature at the second exothermal peak, exhibits Curie temperatures of the Nd2 Fe14 B and Fe3 B phases within the measuring temperature range Meanwhile, the Curie temperature of a (Nd,Dy)2 Fe14 B phase can be found in the thermomagnetic curve of the sample annealed at 670 ◦ C, which is the onset crystallization temperature of the second exothermal peak 120 Figure Temperature dependence of magnetization of annealed sample No measured in 100 Oe applied field Therefore, we suggest that the crystalline evolution process in our materials is as follows: Amorphous → Amorphous’ + Fe3 B → Amorphous” + Fe3 B + Nd2 Fe14 B → Amorphous”’ + α-Fe + (Nd,Dy)2 Fe14 B → α-Fe + Fe3 B + (Nd,Dy)2 Fe14 B Crystalline evolution and coercivity in nanocomposite magnets Table Magnetic parameters for sample No at differing annealing temperatures Ta (◦ C) Mr (emu g−1 ) Mr /Mmax B Hc 660 670 680 690 700 710 106 126 123 127 127 121 0.64 0.73 0.70 0.73 0.74 0.71 1370 3400 3500 3580 3660 3520 (Oe) (BH)max (MGOe) 9.5 12.6 12.3 15.0 15.9 13.4 Table Magnetic parameters for sample No at different annealing temperatures Figure Magnetic parameters as a function of annealing temperature for sample No (annealing time of min) Ta (◦ C) Mr (emu g−1 ) Mr /Mmax B Hc 640 650 660 670 680 690 126 128 126 124 121 120 0.76 0.78 0.77 0.76 0.76 0.73 2110 3050 2980 2920 2840 2600 (Oe) (BH)max (MGOe) 10.5 16.2 15.4 14.4 13.3 12.0 remanence as well as reduced remanence (see figures and and tables and 2) The value 4.2 kOe for No is quite a high achievement obtained so far for nanocomposite magnets with low rare-earth contents Figure Annealing time dependence of magnetic parameters for sample No after annealing in Figures and show annealing temperature dependence of magnetic characteristics of the samples derived from VSM First of all, coercivity and remanence of both samples gradually increase with annealing temperature and after that they drastically increase to large values This could be explained as follows: • At an annealing temperature, which is lower than the temperature of the second exothermal peak, there is the crystallization of the Fe3 B phase and a small amount of the Nd2 Fe14 B phase The volume fraction of Nd2 Fe14 B increases, leading to an increase in coercivity; • As the annealing temperature increases to the temperature at the second exothermal peak, Dy atoms replace the Nd ones in the crystal lattice of the 2: 14: phase to form the (Nd,Dy)2 Fe14 B phase In the Dy2 Fe14 B, which has twice larger magnetocrystalline anisotropy than that of Nd2 Fe14 B [18], there is a dramatic increase in coercivity (see inset in figure 2) Besides the increase in the volume fraction of hard phases, a strong exchange coupling between the soft and the hard phases leads to an increase in Microstructural observation was performed for the annealed samples Figure is a typical example for this measurement We can say that, in sample No 2, the grain size is always smaller than that of sample No For example, in figure 5, the average size of nano-crystallites is 45 nm for sample No (after optimally annealing) whereas this value is 27 nm for sample No In sample No 2, there is a co-doping of Nb and Cu This produces a well-known effect in that Cu promotes nucleation in the crystallization process, and Nb plays a role in retarding the growth of the crystal grains [5, 17] Copper atoms form a high density of clusters prior to the crystallization reaction, which serve as nucleation sites for the bcc-Fe primary crystals Niobium added in combination with Cu induces the formation of the Nd2 Fe14 B and metastable phases in the second stage of the crystallization process by partitioning in it Because two phases are formed from the remaining amorphous phase, the crystal grain size in the final microstructure becomes smaller than that of the specimen without Nb and Cu So, the co-doping of Cu and Nb creates a grain refinement, which enhances the exchange coupling between magnetically hard and soft nano-grains (see table 2) Enhancement of exchange coupling causes a highly reduced remanence up to 0.78 (for sample No 2) at the optimal annealing condition Conclusion The crystalline evolution, and magnetic properties of (Nd,Dy)2 Fe14 B/α-Fe nanocomposite magnets with low rareearth contents have been investigated A small amount of Dy substitution for Nd leads to an enhancement in the coercivity of the materials, up to 4.2 kOe This value is much larger than that of similar compositions reported previously by other authors [16] The effect of Cu/Nb co-doping on microstructural refinement is discussed 121 N D The et al for Natural Sciences (Project 406506), and research at Chungbuk National University was supported by the Korean Science and Engineering Foundation through the Research Center for Advanced Magnetic Materials at Chungnam National University References Figure SEM micrographs of optimally annealed samples No (a) and No (b) Acknowledgments Research at Center for Materials Science, VNU, is financially supported by the Vietnamese Fundamental Research Program 122 [1] Coehoorn R, de Mooji D B and de Waard C 1989 J Magn Magn Mater 80 101 [2] Xiong X Y, Hono K, Hirosawa S and Kaneikiyo H 2002 J Appl Phys 91 9308 [3] Zhang X Y et al 2002 Phys Rev B 66 212103 [4] Lin Z Q, Cui B Z, Liu J P, Ding Y, Wang Z L and Thadhanni N N 2004 Appl Phys Lett 84 4382 [5] Chau N, Chien N X, Hoa N Q, Luong N H, Niem P Q, Tho N D and Hiep V V 2004 J Magn Magn Mater 282 174 [6] Betancourt I and Davies H A 2005 Appl Phys Lett 87 162516 [7] Chu K-T, Jin Z Q, Chakka V M and Liu J P 2005 J Phys D: Appl Phys 22 4009 [8] The N D, Chau N, Vuong N V and Quyen N H 2006 J Magn Magn Mater 303 e419–22 [9] Zeng H, Li J, Liu J P, Wang Z L and Sun S 2002 Nature 420 395 [10] Sui Y C, Liu W, Yue L P, Li X Z, Zhou J, Skomski R and Sellmyer D J 2005 J Appl Phys 97 10J304 [11] Zhang W, Louzguine D V and Inoue A 2004 Appl Phys Lett 85 4998 [12] Chang C W, Chang H W, Chiu C H and Chang W C 2005 J Appl Phys 97 10N117 [13] Liu W, Zhang Z D, Liu J P, Dai Z R, Wang Z L, Sun X K and Sellmyer D J 2003 J Phys D: Appl Phys 36 L63 [14] Chang H W, Chiu C H and Chang W C 2003 Appl Phys Lett 82 4513 [15] Zhang W Y, Chang H W, Chiu C H and Chang W C 2004 J Alloys Compounds 379 28 [16] Li S, Gu B X, Sen Y, Hong B, Yaodong D, Zongjun T, Guozhi X, Youwei D and Zuanru Y 2002 J Phys D: Appl Phys 35 732 [17] Ping D H, Hono K, Kanekiyo H and Hirosawa S 1999 Acta Metall 47 4641 [18] Sagawa M, Fujimura S, Yamoto H, Matsuura Y and Hiraga K 1984 IEEE Trans Magn 20 1584 ... that the crystalline evolution process in our materials is as follows: Amorphous → Amorphous’ + Fe3 B → Amorphous” + Fe3 B + Nd2 Fe1 4 B → Amorphous”’ + α -Fe + (Nd,Dy)2 Fe1 4 B → α -Fe + Fe3 B + (Nd,Dy)2... → α -Fe + Fe3 B + (Nd,Dy)2 Fe1 4 B Crystalline evolution and coercivity in nanocomposite magnets Table Magnetic parameters for sample No at differing annealing temperatures Ta (◦ C) Mr (emu g−1... heating rate of 20 ◦ C min−1 measured in flowing Ar gas (a) and XRD results for sample No at different annealing temperatures (b) • The first peak corresponds to the crystallization of the Fe3